1. Field of the Invention
The present invention relates to a micro-actuator array, and a micro-actuator device, an optical switch array, and an optical switch system, which use the micro-actuator array. Such an optical switch array may be used in, for example, optical communication.
2. Description of the Related Art
Development in micromachining technology has increased the importance of actuators in various fields. A microactuator may be used in, for example, the field of an optical switch used in, for example, optical communication and used for switching optical paths. An example of such an optical switch is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2001-42233.
A microactuator for moving a micromirror is used in the optical switch disclosed in Japanese Unexamined Patent Application Publication No. 2001-42233. The microactuator comprises a movable section which is movable with respect to a fixed section. The movable section can be restored to an upper position (where the micromirror reflects incident light) by its springiness. A first electrode (fixed electrode) is disposed at the fixed section, and a second electrode (movable electrode) is disposed at the movable section. By applying a voltage between the first and second electrodes to generate an electrostatic force therebetween, the movable section is moved to a lower position (where the micromirror transmits incident light as it is) and held there. When the application of the voltage between the first and second electrodes is stopped, the movable section is restored to the upper position by its springiness.
Another example of an optical switch is a matrix optical switch disclosed in Japanese Unexamined Patent Application Publication No. 5-113543. In the optical switch, very small mirrors are driven by a plurality of microactuators disposed in a matrix on the same substrate. The driving principle of each microactuator is, for example, electromagnetic induction. In addition, in the optical switch, for example, a selective circuit and address circuits (X address decoder and Y address decoder) for supplying a drive signal to each microactuator by decoding a command signal are disposed on the substrate.
Optical switches are available in various sizes, from small sizes of 2×2 channels and 4×4 channels to large sizes of 128×128 channels and 256×256 channels. Japanese Unexamined Patent Application Publication No. 2001-42233 only discloses a one-channel optical switch. As in Japanese Unexamined Patent Application Publication No. 5-113543, the size of this optical switch can be increased by disposing a plurality of the microactuators disclosed in Japanese Unexamined Patent Application Publication No. 2001-42233 on the same substrate.
When an optical switch is formed by disposing a plurality of microactuators using electrostatic force, the wiring method differs depending upon the size of the optical switch.
Although not disclosed in Japanese Unexamined Patent Application Publication No. 2001-42233 or Japanese Unexamined Patent Application Publication No. 5-113543, in a small optical switch, conventionally, either one of the fixed electrodes and the movable electrodes of the respective microactuators are electrically connected in common on a substrate and are defined as common electrodes, whereas the other of the fixed electrodes and movable electrodes of the respective microactuators are used as individual electrodes without being electrically connected to each other. These electrodes are connected outside the substrate. Therefore, for example, for an N×N channel optical switch, N2+1 wires are connected outside the substrate. The phrase “electrodes are connected outside the substrate” refers to electrically connecting an external connection wiring to the electrodes. For convenience of explanation, the aforementioned method will be called a “common electrode/individual electrode external connection method.”
In contrast, in a large optical switch, conventionally, as disclosed in Japanese Unexamined Patent Application Publication No. 5-113543, for example, an address circuit and a selective circuit are formed using, for example, CMOS on a substrate on which the microactuators are mounted, and about ten wires are connected outside the substrate. For convenience of explanation, such a method will be called an address circuit mounting method.
The address circuit mounting method is very advantageous from the viewpoint that a small number of wires are connected outside the substrate even if the optical switch is large. However, the address circuit mounting method has the following first to third disadvantages. Therefore, if the optical switch is not at least a certain size, the disadvantages far outweigh the advantages. As a result, the address circuit mounting method is not suitable for optical switches of intermediate and smaller sizes.
The first disadvantage of the address circuit mounting method is that highly pressure-resistant MOS is required for forming, for example, an address circuit and a selective circuit because a voltage of about tens of volts, or in some cases a high voltage equal to or greater than 100 volts, is often required for electrostatically driving an actuator. Since a device formed of a highly pressure-resistant MOS has a larger planar size than that formed of an ordinary MOS, the size of the actuator is also increased. The second disadvantage is that costs are increased because the number of steps of a production process is increased by a MOS production step. The third disadvantage is that costs are increased due to the addition of a smoothening step, which is carried out because, if smoothening is not sufficiently carried out after producing MOS, the shape of a base is transferred to the shape of MEMS (Micro Electro Mechanical System) to be formed on top of MOS, causing malfunctioning.
In contrast to this, the common electrode/individual electrode external connection method does not have the disadvantages of the address circuit mounting method because an address circuit, a selective circuit, etc., are not mounted. However, even if the optical switch is of an intermediate or smaller size, the optical switch becomes only slightly larger, with the number of wires to be externally connected being considerably increased. Therefore, the optical switch can be formed with a size of the order of 8×8 channels at most. Even if the optical switch is an 8×8 channel optical switch, it is necessary to externally connect as many as 65 wires.